Interpretive Summary: If biofuel crops are grown in drought-prone soils to reduce food crop competition, climate change may increasingly limit productivity of biofuels by decreasing soil moisture. We ask whether the addition of biochar to soils can significantly increase the availability of water to a crop. Biochar addition significantly increased the available water content of the soils by both increasing the amount of water held at field capacity and by allowing plants to draw water to a lower concentration in the soil before wilting. We found that biochar amendment of soils increased available soil water equivalent to 0.8 to 2.7 d of switchgrass transpiration, which could help mitigate predicted decreases in water availability.

Technical Abstract:
Because of climate change, insufficient soil moisture may become an increasing limitation to crop productivity in certain regions of the world. This may be particularly consequential for biofuel crops, many of which will have to be grown in drought-prone soils to avoid competition with food crops. Biochar is the byproduct of a biofuel production method called pyrolysis. If pyrolysis becomes more common as some scientists predict, biochar will become more widely available. We asked, therefore, whether the addition of biochar to soils can significantly increase the availability of water to a crop. Biochar made from switchgrass (Panicum virgatum L.) shoots was added at the rate of 1% of dry weight to four soils of varying texture, and available water contents were calculated as the difference between field capacity and permanent wilting point water contents. Biochar addition significantly increased the available water contents of the soils by increasing the amount of water held at field capacity and allowing plants to draw water to a lower concentration in the soil before wilting. Among the four soils tested, biochar amendment resulted in an additional 0.8 to 2.7 d of full transpiration, which could result in a significant increase in total photosynthesis. Biochar amendment of soils may thus be a viable means of mitigating some of the predicted decrease in water availability accompanying climate change that could limit the future productivity of biofuel crops.